Anicca (Sanskrit anitya), impermanence, is the first of the three marks of existence: everything conditioned arises, changes and passes away. The claim is stronger than the observation that things end. It is that conditioned things are processes throughout, with no persisting substance underlying the change, what appears to be a thing is a sequence of arisings, and the appearance of a stable object is produced by the rapidity and similarity of the succession.
The teaching is stated constantly in the canon, and the tradition characteristically attaches a practical conclusion to it rather than leaving it as metaphysics. The verse formula "all conditioned things are impermanent" continues, in the account of the Buddha's last words in the Mahaparinibbana Sutta, with the instruction to strive on with diligence; the point of the observation is what one does about it.
Anicca is doctrinally load-bearing because the other two marks follow from it. If what a person consists of is impermanent, then clinging to it produces dukkha, unsatisfactoriness; and if nothing among the constituents persists, there is no permanent self to be found among them, which is anatta. The Anattalakkhana Sutta, traditionally the Buddha's second discourse, argues in exactly this order through the five aggregates, asking of each in turn whether it is permanent, whether what is impermanent is satisfactory, and whether what is impermanent and unsatisfactory can properly be regarded as "mine, I, my self".
Abhidharma pressed the analysis into the doctrine of momentariness: phenomena last only an instant and are replaced by successors, so that continuity is a sequence rather than a persistence. This generated a long internal debate about how causation, memory and moral responsibility can work if nothing lasts, and the schools answered differently, the Sarvastivadins holding that dharmas exist in past, present and future in some sense, and others rejecting that as covertly reinstating permanence.
Meditatively, the direct perception of arising and passing away is treated as a specific stage of insight practice rather than a proposition to be assented to, and the Theravada commentarial tradition maps its stages in detail. The doctrine also grounds a whole aesthetic of transience, most visibly in the Japanese sensibility of mono no aware and in the cultural weight given to the cherry blossom, an unusually direct case of a metaphysical claim producing a style of attention.